Support retrieving current term

[otoolep]

Simple addition to make this internal variable accessible (without relying on stats object, which is a brittle approach).

Term is a fundamental concept in Raft consensus, so making it easily available to clients of this library seems like an obvious thing to do. There are also specific use cases supported by knowing the Term:

• Allow clients of this library to retrieve and display diagnostic information about the state of the Raft system.
• Support certain types of reads of a Raft-managed store. For example, if a client can check before and after a Read that an election has not taken place during the read, it allows those clients to make certain guarantees about the data read from the Raft-managed Store. One way to do this is to simply check that the Term has not changed (see this discussion for more details).

[otoolep]

I need this for my work to add Linearizable reads to rqlite, version 8.32.0. See https://github.com/rqlite/rqlite/blob/master/CHANGELOG.md

This seems like something the library should expose, perhaps no one needed it yet.

[schmichael]

Looks good to me, but I'm going to hold off slapping merge until @rboyer or somebody else gets a chance to weigh in.

I'd personally love a Why in the PR description. Skimming the linked mailing list thread gave me enough context to 👍 this, but a nice succinct summary here would be nice for any future generations hoping to figure out when Term is useful. Not a blocker though!

[otoolep]

Description updated -- thanks.

[schmichael]

Merged, thanks for your contribution and patience @otoolep !